We discuss recent work exploring the use of CMB lensing to probe E_G,
the ratio between curvature and velocity perturbations. This quantity is
distinct for various gravity models, breaking the degeneracy in current
cosmological probes of gravity and dark energy. While the lensing
signal within E_G has traditionally been probed with galaxy-galaxy
lensing, galaxy-CMB lensing is a more robust lensing tracer that can
probe E_G at higher redshifts with fewer astrophysical uncertainties. We
present constraints to E_G using CMB data from Planck and galaxy data
from the SDSS BOSS survey, which are consistent with general relativity
(GR). We also show forecasts for future surveys, finding that upcoming
photometric surveys combined with next-generation CMB surveys like
Advanced ACTPol could produce <1%-level constraints that could
potentially rule out alternatives to GR.